TECHNICAL INFORMATION ON CHELATE STABILITY

The stability (tendency to dissociate in solution) of metal complexes depend on both the metal ion and the ligand:

-  Metal ion  - increases stability of the complex with:

•  Increasing ion charge - metals have different charges depending on their oxidation state  E.g. Mg2+, Fe(II) 2+ (Ferrous), Fe(III) 3+ (Ferric), Zn2+, Cu(I)+ (Cuprous), Cu(II)2+ (Cupric)

•  Decreasing size

•  Increasing electron affinity

-  Ligand - factors increasing the stability of the complex:

•   Basicity of the ligand - coordination compounds are formed as a result of acid-base reactions, where the metal ion is the acid and the ligand is the base

 •  Number of metal-chelate rings per ligand

 •  Size of the chelate ring

 •  Steric effects

 •  Resonance effects

Note that the percentage of a metal formed /present as a particular complex in solution depends on:

-   Its environmental pH

-   The  solubility constant of the complex - which is affected by temperature

Stability Constants of Metal Complexes

A measure of the strength of the reaction between the specific metal with the specific ligand in solution. Values range from 0-20 - the lower the number, the weaker the strength of the reaction, and the more likely the metal will separate / dissociate from its ligand in solution.

Negative Values - 1:   metal-ligand is soluble in water and will readily dissociate yielding all of the metal in ionic form in pH levels of 2 (stomach acid) to 7.4 (extracellular body fluids: serum, lymph) 

Values of 1-3: Metal-ligands are soluble and ~5% ionic at pH 7.4

Values 3- ~6: metal-ligand will likely dissociate in stomach acid, but not in physiological pH 7.4

Values >6: less and less metal-ligand dissociates at any pH and such compounds are unusable by the body  

Dissociation ability is inversely affected by pH and temperature - the stomach acid has a pH of 2-3, duodenum (pH 6), increasing to pH 7.4 in the terminal ileum. The pH drops to 5.7 in the caecum, but again gradually increases, reaching pH 6.7 in the rectum. Ref

Amino Acids

Amino acids have three basic parts: the amino ‘group’ (i.e., group of atoms), the acid group, and the R-group - It is the R-group that determines the name and specific character of an amino acid -determining, for example, whether the amino acid is aspartic acid or lysine or tryptophan.